Increasing the sensitivity of light-sensitive silver halide photographic emulsions is the most effective means of improving various characteristics of photographic light-sensitive materials. For example, high-speed color photographic light-sensitive materials currently used have been realized by increasing the sensitivity of photographic emulsions. With respect to image quality, it is also well known that graininess can be improved by the use of smaller silver halide grains with an enhanced sensitivity. Further, in the manufacture of radiographic light-sensitive materials, a technique for improving the sensitivity of light-sensitive silver halide photographic emulsions is not dispensable for securing a desired sensitivity with the sharpness kept high by cutting down crossover light. Therefore, various studies have so far been made in the industry for the purpose of raising the sensitivity of light-sensitive silver halide photographic emulsions.
In recent years, there are disclosed a variety of techniques which use tabular silver halide grains for the purpose of raising the sensitivity, and examples thereof can be seen in Japanese Pat. O.P.I. Pub. Nos. 111935/1983, 111936/1983, 111937/1983, 113927/1983, 99433/1984, etc.
Further, Japanese Pat. O.P.I. Pub. No. 92942/1988 discloses a technique to provide cores of high silver iodide content inside tabular silver halide grains, and Japanese Pat. O.P.I. Pub. No. 151618/1988 discloses a technique which uses hexagonal tabular silver halide grains; favorable results are reported on both the techniques.
Furthermore, techniques relating to the composition distribution of tabular silver halide grains are disclosed in Japanese Pat. O.P.I. Pub. Nos. 106746/1988, 183644/1989 and 279237/1989. With respect to the crystal structure of tabular silver halide grains, there are disclosed several techniques which relate to the form or parallel twin planes of tabular grains. For example, Japanese Pat. O.P.I. Pub. No. 131541/1989 discloses a technique to improve the sensitivity and graininess by use of discoidal grains.
Japanese Pat. O.P.I. Pub. No. discloses a technique using tabular silver halide grains having two or more parallel twin planes in which the ratio of intertwin-plane distance between parallel twin planes (a) to grain thickness (b), or (b/a), is 5 or more and describes the effect on the sensitivity and graininess; particularly, a technique to increase the uniformity of intertwin-plane distances of grains, and the enhancement in sensitivity and the improvement in graininess thereby attained are described.
Wo No. 91/18320 discloses a technique using tabular silver halide grains whose intertwin-plane distances (a) are 0.012 .mu.m or less and describes that a desirable high sensitivity has been attained by this technique.
EP No. 515894A1 discloses the achievement of high sensitivity by making the percentage of (111) faces in side-face of silver halide grains, having a tabularity given by (grain diameter)/(grain thickness).sup.2 of 25 or more, 75% or less.
On the other hand, there have been disclosed various techniques to eliminate the defects of tabular silver halide grains. For example, Japanese Pat. O.P.I. Pub. No. 142439/1991 discloses a technique for improving the preservability under highly humid conditions by use of an emulsion in which 50% or more of the total projected area comes from tabular grains having an aspect ratio of 3 or more and having (111) faces and (100) faces.
Since these tabular silver halide grains are larger in surface area than silver halide regular crystal grains, such as hexahedral or octahedral crystal grains, when compared in the same volume, sensitizing dyes can be adsorbed in larger amounts on the surfaces of these grains; therefore, it is thought that this brings about advantages of high sensitivity and high sharpness due to decreased scattered light.
However, even when the amount of a sensitizing dye is increased in proportion to the surface area of tabular grains, the sensitivity cannot be raised so much as expected in fact; further, stains attributable to residual dyes are liable to occur because of shortening of developing time. Furthermore, organic solvents and/or surfactants needed for adding dyes in large amounts are liable to cause troubles such as formation of precipitates in a silver halide photographic emulsion or coating failures including spots and streak lines in the process of coating emulsions. In addition, the use of organic solvents poses problems in operation and environmental protection.
In incorporating sparingly water-soluble photographic additives into a silver halide photographic emulsion, the usual method comprises the steps of dissolving a photographic additive in an organic solvent such as methanol and then adding the solution to a silver halide photographic emulsion. Instead of this conventional method, there are attempted in recent years to add an additive by the steps of dispersing the additive, without the aid of an organic solvent, in an aqueous system in the presence of a wetting agent and a dispersing agent and then adding the resultant aqueous dispersion to a silver halide photographic emulsion. For example, Japanese Pat. O.P.I. Pub. No. 110012/1977 discloses such a method, in which a sensitizing dye is ground in an aqueous phase in the presence of a dispersing agent (a surfactant) capable of providing a prescribed surface tension, the resultant aqueous dispersion is dewatered, dried and added to a silver halide emulsion as it is or after being dispersed in water or an aqueous solution of gelatin.
Japanese Pat. O.P.I. Pub. No. 102733/1978 discloses a method comprising the steps of preparing a uniform mixture (a paste-like mixture) containing a photographic fine particle additive, a dispersing agent such as sorbitol and a protective colloid such as gelatin, forming the mixture into noodles, drying them in warm air, followed by granulation. The resulting granules are added to a photographic aqueous colloid coating composition.
Further, U.S. Pat. No. 4,006,025 discloses a method in which a spectral sensitizer is mixed with water to form a slurry, the spectral sensitizer is uniformly dispersed in water by homogenizing or milling at a temperature of 40.degree. to 50.degree. C. in the presence of a surfactant, and then the dispersion so prepared is added to a silver halide photographic emulsion.
Any of them is a method of adding a photographic additive, such as a spectral sensitizer, by use of an aqueous system as a substitute for an organic solvent; but, these show the following disadvantages when put in practical use. Since an aqueous dispersion is made into powder by freeze drying or the like, it takes a long time to have an additive such as a spectral sensitizer adsorbed by silver halide grains; therefore, desired photographic sensitivities cannot be obtained in the usual sensitizing time and, moreover, coating failures attributable to deposits are liable to occur when such a silver halide photographic emulsion is used in coating. Further, a wetting agent and a dispersing agent used in dispersing the additive produce undesired effects such as break of emulsified matters contained in a silver halide photographic emulsion, increased coating failures in high-speed coating of a silver halide photographic emulsion, and low adhesion between coating layers in a manufactured silver halide photographic light-sensitive material.
Further, inferior pressure characteristics (or pressure resistance) are known as another shortcoming of tabular silver halide grains. The term "pressure characteristics" is intended to include pressure fogging which indicates development of unexposed portions and pressure desensitization which indicates lowering in sensitivity, each of which is caused when pressure is applied to a silver halide photographic light-sensitive material. Serious defects may develop in a photographic light-sensitive material when these characteristics are inferior. Generally, silver halide grains are susceptible to pressure and become more susceptible as the sensitivity is raised, and such a tendency is particularly remarkable in tabular silver halide grains. This is attributed to the fact that since tabular grains are subjected, for their thinness, to a moment larger than spherical grains when these grains are the same in volume and, as a whole, the mechanical strength of tabular grains becomes weaker even when the material of tabular grains is the same as that of spherical grains.
Besides the form of silver halide grains, these pressure characteristics also depend upon the silver halide composition of these grains and the conditions of chemical sensitization. Generally, a poor chemical sensitization (a poor chemical ripening) causes a large pressure desensitization, and an excessive chemical sensitization, though it reduces the pressure desensitization, intensifies the pressure fogging. When high iodide content portions are present inside silver halide grains, the pressure fogging tends to decrease but the pressure desensitization is apt to increase.
As preventive measures against deterioration in these pressure characteristics, there have been disclosed various means in Japanese Pat. O.P.I. Pub. Nos. 99433/1984, 301937/1988, 149641/1988, 106746/1988, 151618/1988, 220238/1988, 131541/1989, 193138/1990, 172836/1991 and 231739/1991; but, any of these means is not effective in producing desirable results.
In addition, the above Japanese Pat. O.P.I. Pub. Nos. 163451/1988, 131541/1989, WO No. 91/18320 and EP No. 515894A1 contain neither description suggesting relations between the distance between parallet twin planes or its variation coefficient and aging stability or pressure characteristics, nor description suggesting improvements in such stability and characteristics.